There is no check if the imported file has a default.

Create-index is pretty dumb actually, it would try to include a file as default only if it exists and validates some naming convention. It may eventually import as default a file without default.

Start by locating the logic that generates the ES6 ./index.js file and handles default-file naming; inspect how imported modules are checked. Done means generated indexes do not treat a file as a default export unless it actually has one, with coverage for the case described.

Written by the indexing model from the issue text.